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Plant pathogens and light. Pathogens evolved amidst natural cycles of light and darkness. Electric lighting is a relatively new phenomenon in our world, and especially in theirs. Pathogens sense, interpret and use light to direct their development. In the case of UV radiation, pathogens have evolved defense mechanisms that are regulated by visible light. We seek to exploit these evolved relationships as a novel and effective means to suppress plant diseases and promote plant health.

We work as a diverse international group to promote this research area and its applications, and to act as a resource to train others. The group includes Rensselaer Polytechnic Institute's Lighting Research Center (RPI/LRC), Norway's Institute of Bioeconomy Research (NIBIO), the Norwegian University of Life Sciences (NMBU), the University of Florida Gulf Coast Research and Education Center (UFL/GCREC), and Cornell University's Geneva Experiment Station (Cornell/Geneva). The work spans disciplines from plant growth and photobiology to physics and lighting technology.

Support for research. Our work has been generously supported by grants from the USDA Organic Research and Extension Initiative (OREI), the Specialty Crops Research Program (SCRI), The Research Council of Norway (RCN), and the New York Farm Viability Institute, as well as by assistance from the lighting companies OSRAM, Ushio, Cree, and the Asahi Glass Company.

What is the problem?

• Plant pathogens respond to optical radiation in ways that, with few exceptions, are just beginning to be understood.

• By not understanding the impacts of optical radiation in pathosystems, we forgo opportunities to lessen severity of disease, and in some instances we exacerbate disease.

• There is a critical need in IPM programs for alternative methods to suppress plant pathogens both to preserve genetic resistance to disease, and to lessen selection for pathogen resistance to fungicides.

What have we discovered?

Research and demonstration greenhouses at Cornell's Experiment Station and at the USDA Grape Genetic Research Unit in Geneva, NY. These automated night-operated lighting systems employ both UVB lamps and specific LEDs to suppress powdery mildews, photosensitive downy mildews, and arthropod pests such as spider mites. (Click for larger image.)

• UV-B and UV-C can be used at greatly reduced doses when applied at night to effectively suppress certain plant pathogens.

• Recent results suggest that nighttime UV treatments are ovocidal and suppress phytophageous mites.

• LEDs producing specific wavelengths in the visible range can be used to target photosystems in diverse pathogen groups (e.g., Erysipaceae and Oomycetes) and disrupt fundamental processes such as sporulation.

• We can design lamp arrays to fit canopy characteristics and ground speed requirements for commercial field applications of UV.

• We can design static, mobile, and fully autonomous systems for use in glasshouses or high tunnel production systems.

What is the basic science?

Microscopic view of a powdery mildew colony on a strawberry leaf. The pathogen grows externally on plant surfaces, sending absorptive structures into the epidermal cells. It's niche makes it especially vulnerable to targeted light treatments. (Click for larger image.)

• Pathogen sporulation may be limited or accelerated in response to visible light, circadian effects, or the interaction between the two.

• UV wavelengths between 250 and 280 nm are equally damaging to pathogen DNA, but the longer wavelengths near 280 nm are less damaging to host plants.

• Pathogen repair of DNA is upregulated by blue light and UV-A and downregulated by red light and during darkness.

• Both pathogen and host sensitivity to UV is conditioned by the daily light integral.

What are the existing technologies?

• UV-B lamps produce UV radiation at 280 nm, the most desirable wavelength for killing pathogens with minimum harm to host, but their relatively low power in this region of the spectrum makes them better suited for static arrays or those used on slow-moving (e.g., 50 cm/min) booms, or autonomous mobile systems.

Video clip of mobile UV-C array drawn by tractor. (Click to download .mov video.)

• UV-C lamps produce a narrowband output centered on 254 nm. Their relatively high power makes it possible to design lamp arrays that can be used in mobile tractor-drawn arrays for large field applications.

• LEDs produce narrowband light and are thus ideal for applications requiring specificity of wavelength or spectral distribution. Arrays can also be created in many wavelength combinations. LEDs are solid-state devices and provide an unprecedented degree of control of intensity, spectral distribution, and the ability to pulse and counterphase, which has exciting possibilities for pathogen suppression and plant modification.

What are the present applications and implications for the future?

Plastic tunnels for strawberry culture at the University of Florida. These tunnels are constructed of a special fluorocarbon plastic that uniquely transmits nearly 100% of incoming UV light. This greatly reduces severity of powdery mildew, which has historically plagued tunnel production systems for many crops. (Click for larger image.)

• Use of electric lighting in ways that complement rather than complicate disease suppression, particularly in controlled environment agriculture.

• Expansion of opportunities to use light to suppress arthropod pests.

• Design of glasshouse and tunnel systems based upon the pest- and disease-relevant optical properties of materials (e.g., epidemiological consequences of fluorocarbon vs. polyethylene plastics).